This is a very carefully crafted text. And there's a reason for that. Abraham is an extreme example of the kind of situation all will eventually face as they place trust in the good promises of God. The important words are in verses 17b and 18 - "...and he who had received the promises was in the act of offering up his only son,  of whom it was said, Through Isaac shall your offspring be named.' "
Notice that description of Isaac as Abraham's "only son"(17). In other words, Ishmael and any other sons didn't count. God had been very specific with His promise to Abraham - Genesis 21:12 - "But God said to Abraham, 'Be not displeased because of the boy and because of your slave woman. Whatever Sarah says to you, do as she tells you, for through Isaac shall your offspring be named.' "
Isaac was unique and irreplaceable in the plan of God, and God had told as much to Abraham - Genesis 22:2 - "He said, 'Take your son, your only son Isaac,whom you love, and go to the land of Moriah, and offer him there as a burnt offering on one of the mountains of which I shall tell you.' "
Don't miss the deliberate tension in our Hebrews text. Our inspired writer very carefully and specifically identifies Abraham, "...he who had received the promises..."(17). And those promises included descendants like the stars in the heavens and the sands on the seashore through Isaac. This is the Abraham being asked to offer up his son Isaac. He is specifically identified as the receiver of divine promise. That's what Abraham is famous for to this day. That's how we all identify him. He's the one with whom God made these covenant promises through Isaac.